Method for enhancing the removal of volatile species from liquids

ABSTRACT

A method for effecting the disintegration of a liquid into exceedingly fine droplets, thereby effecting an increase in the surface area of the liquid with a concommitant increase in the removal rate of volatile impurities. A non-deleterious gas is injected in the form of bubbles into the liquid, prior to its passage through an orifice into a region of lower pressure, whereby the expansion of the gas bubbles causes the liquid streaming through the orifice to spread radially and disintegrated.

United States Patent Olsson et al.

[451 Feb. 5, 1974 METHOD FOR ENHANCING THE REMOVAL OF VOLATILE SPECIES FROM LIQUIDS [75] Inventors: Robert G. Olsson, Edgewood Borough; Ethem T. Turkdogan, Pittsburgh, both of Pa.

[52] U.S. Cl 75/49, 75/59, 75/60, 159/48, 203/49 [51] Int. Cl. C2lc 7/10, B01d 1/16, F26b 3/12 ['58] Field of Search.... 75/49, 59; 159/48 R; 203/49 [56] References Cited UNITED STATES PATENTS 3,145,095 8/1964 Franzen 75/49 3,031,261 4/1962 Vogel et a1 159/48 R 860,929 7/1907 Merrell et a1. 159/48 R 3,606,291 9/1971 Schneider 75/49 X 424,756 4/1890 Blackman 159/48 R 1,406,381 2/1922 Heath et a1. 159/48 R 3,166,613 1/1965 Wright et a1. 159/48 R 3,615,723 10/1971 Meade 159/48 R X 1,323,583 12/1919 Earnshaw", 75/60 X 327,419 9/1885 Witherow 719,725 2/1903 Bertou 75/0.5 C

FOREIGN PATENTS OR APPLICATIONS 930,018 7/ 1963 Great Britain 75/49 OTHER PUBLICATIONS Perry; Chemical Engineers Handbook; 4th Edition; 1963; page 18-66.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm Arthur Greif [5 7] ABSTRACT A method for effecting the disintegration of a liquid into exceedingly fine droplets, thereby effecting an increase in the surface area of the liquid with a concommitant increase in the removal rate of volatile impurities. A non-deleterious gas is injected in the form of bubbles into the liquid, prior to its passage through an orifice into a region of lower pressure, whereby the expansion of the gas bubbles causes the liquid streaming through the orifice to spread radially and disintegrated.

6 Claims, 9 Drawing Figures t 70 VACUUM r- PUMP PAIENIEDFEB 51914 3.790.368

sum 1 BF 4 F/G. FIG. 3.

70 VA CUUM PUMP GA 8 70 VA CUUM PUMP III 7'0 VACUUM PUMP INVENTORS.

ROBERT G. OLSSO/V 8 ETHEM 7'. TURKDOGA/V A lorney PAIENTEU 74 SIEUEBfd FIG. 4

IIVI/E/VTOHS.

ROBERT G OLSSON 8 THQW r TURKDOGAN 1 44M 1 Affor METHOD FOR ENHANCING THE REMOVAL OF VOLATILE SPECIES FROM LIQUIDS This invention relates to a method for effecting the extraction of volatile species from a liquid.

Low pressure flashing to effect a separation of a volatile component of a liquid has been employed in such diverse areas as distillation, solvent recovery, dehydra-' hancing the desired removal. It has now been discovered that if a small amount of gas is dispersed in the form of spaced-apart bubbles into the liquid so as to be entrained in the stream, prior to the passage of the liquid stream through a nozzle of reduced crosssectional area and its entrance in the region of lowered pressure, these bubbles of gas will cause the stream to break up into a spray of fine droplets. Gas injected in this manner will cause the stream to disintegrate even when there is no dissolved gas evolution from the liquid.

The objects and advantages of the invention will be better understood by reference to the appended claims and the following descriptions and Figures in which:

FIG. 1 shows an apparatus employed in the prior art mechanical lift processes using gas injection;

FIG. 2 is a modified stream degassing apparatus employing the gas injection of the instantinvention;

FIG. 3 diagrammatically depicts an apparatus useful for a further embodiment of the invention;

FIG. 4 is a diagram of the apparatus employed in the study of oil sprays;

FIGS. 5A and 5B show the effect of the instant method on the disintegration of oil sprays;

FIG. 6 is a graph of the experimental results showing the correlation with equation 1;

FIG. 7 depicts the percentage of O removed as a function of gas injection rate; and

FIG. 8 is a graph which shows the percentage 0 removed as a function of vacuum chamber pressure (P for gas injection rates of l/min.

To better understand the operation of the subject process, the invention will be described in its applicability to the degassing of molten metals. The particular improvements and advantages of the instant method may be more fully appreciated by comparison with prior art metal degassing methods which also use bubble injection, i.e., the mechanical lift processes. These processes are, in general, similar to that shown in U.S.

Pat. No. 1,921,060, which discloses an apparatus for conducting molten metal through an inverted U-tube having an inlet leg connected to a first vessel containing the metal to be degassed. The metal is conducted by this inlet leg through a vacuum chamber and then through an outlet leg to a second container in which the degassed metal is collected. Various improvements on this basic process are exemplified by, for example, U.S. Pat. Nos. 2,893,860; 3,042,510; 3,136,834; and 3,321,300.

The basic operation of these prior art systems will be understood by reference to FIG. 1.To effect the motion of the metal (indicated by solid arrows) through the vacuum chamber 1, a low density material, generally a gas, is injected into the molten metal near the lower end of the intake leg 2, by means of a supply pipe 3. By injection of such a low density material, the average density of the molten metal in the intake leg is decreased, thereby causing the level of metal within the intake leg to rise to a higher level and at greater velocity than that achieved by normal barometric action. The degassed metal then flows through the outlet leg 4. This leg may then serve to deliver the degassed metal to a second container (not shown) as stated herein- .above, or to effect acontinuous recirculation of the molten metal until the desired degree of degassing is attained (as depicted in FIG. 1). The injection of this gas serves a second function by exerting a scavenging action, in which the dissolved gases (e.g., CO, H may diffuse into the so-formed bubbles, prior to entrance into the vacuum chamber. However, the primary purpose of the injected gas is to make the apparent density of the liquid in the intake leg lighter and therefore the gas is generally injected as low in the leg as possible. Thus, although the gas bubbles as initially injected, are at a pressure equal to that of the surrounding liquid,

(e.g., l atrn.) their comparativelyslow rise in the up leg results in a great deal of expansion. Consequently, the gas bubbles enter the vacuum chamber at a far reduced pressure, and their capacity to do work, i.e., their stored energy, is substantially reduced.

Contrary to these prior art methods, the instant process (FIG. 2) effects a substantially enhanced disintegration of the molten metal, by causing the metal in vessel 1, with its entrained bubbles, to be suddenly ac-v celerated as it passes through the nozzle of substantially reduced cross-sectional area 2 and into the vacuum chamber 3. where A, represents the cross-sectional area of vessel 1, and A the area of the orifice, it is preferred that the ratio of A /A be at least 2:1. With this acceleration of the liquid metal, there is a corresponding sudden drop in pressure from that at the base of the reservoir to the vacuum chamber pressure. Entrained gas bubbles are rapidly taken from the high pressure region tothe low pressure region where bubble expansion results in the radial spread of the liquid stream and an ensuing spray of fine, large surface area droplets.

To achieve an efficient utilization of gas bubbles for such expansion, the gas must be injected at. a point wherein the liquid is at a pressure considerably higher than that of the reduced pressure region. Thus, the gas may be injected at any point within the containment vessel above the nozzle, where the pressure within the bubbles will be approximately that of the surrounding liquid. However, it is more efficiently injected at a point sufficiently" proximate to the orifice (as' depicted), so as to cause a substantial portion of the bubbles to be entrained within the increment of liquid entering the vacuum chamber.

Thus, referring to FIG. 2, the point of dispersion may be at a region a where the pressure in the liquid is that I of the surrounding atmosphere plus that of the head of liquid h but is preferably at region b where the pressure is even greater (i.e., the atmosphere h,) and a greater portion of the bubbles will be entrained in the liquid entering the nozzle. The length of the nozzle should be short (for purposes of this invention, the term nozzle is meant to include a knife edge orifice, i.e., a device for converting the pressure and potential energy of a fluid into kinetic energy), so that the passage of liquid with its entrained bubbles will be sufficiently rapid, thereby preventing the bubbles from expanding to a substantial extent prior to the time the liquid has left the nozzle. Consequently, substantially all the gaseous expansion will be available for spray formation. The maximum nozzle length will depend on a number of factors. Thus, the diminished expansion caused by a relatively long nozzle could be overcome to some extent by increasing the ambient pressure of the liquid and/or by increasing the force by which the liquid is forced through the nozzle. These latter expedients serve to increase thevelocity of liquid through the nozzle and to increase the initial stored energy in the bubbles. Thus, to prevent such substantial expansion of the bubbles within the nozzle, with its consequent loss of available energy, it is considered desirable to limit the residence time of the incremental portion of liquid within the nozzle. This time limit is basically a function of the pressure'of the liquid at the point of gas injection (and hence the pressure inside the hub bles prior to any expansion). It is, therefore, preferred that the liquid be urged through the nozzle with sufficient force so that the residence time of the liquid within the nozzle is defined by the equation:

t 0.1 V P,

where t residence time in seconds, and

P pressure in atmospheres, of the liquid, at the point of gas injection.

To achieve such desirable short residence times, the liquid metal is preferably accelerated through the nozzle by the application of a force in addition to, and acting in con ert with the atmospheric force. Thus, in FIG. 2, the hea of liquid (gravity) and the atmosphere both aid in forcing the liquid through the nozzle. However, the instant method need not rely on gravitational or atmospheric forces. Thus, for example, a piston device may be employed to force the liquid. into the vacuum chamber (as shown diagrammatically in FIG. 3).

' For additional understanding of its purposes, the invention is described below in its application to two specific embodiments; (l) the extraction of toluene from petroleum oil; and (2) the stream degassing of molten steel.

Extraction of toluene A schematic diagram of the apparatus used in the study of oil-toluene sprays is shown in FIG. 4. The upper portion of the vacuum tank 1 was an 18 in. X 18 in. diameter glass cylinder through which jets could be observed and photographed. The lower portion was a stainless steel container that served as a reservoir from which the liquid was recycled. The desired liquid flow rate was set with a variable speed gear pump 2. Toluene vapor was drawn off by vacuum pump 3, condensed in a cold-finger condenser 4 and collected in a graduated vessel 5, at the base of the condenser. The petroleum oil employed, had a density of 0.81 g/cm, a surface tension of 30.8 dyne/cm and a vapor pressure at 18 C, of less than 0.1 torr. The vapor pressure of the oil-toluene mixture with from 1 to percent toluene followed the relationship P 2.6 (volume percent toluene) 0.32; the extracted vapor being essentially pure toluene. Gas (N was injected 6 in the form of fine bubbles, through either a porous nickel filter or a 0.02 inch hypodermic needle, at a point just above the nozzle opening.

High speed still and motion pictures were taken of many of the so-formed sprays; the exposure time for the still photographs being about 3 X 10 seconds. FIGS. 5A and 5B are facsimile tracings of these photographs showing the effect of bubble injection on the disintegration of the stream. In the oil-toluene mixtures employed, the toluene vapor pressure was never greater than a few torr above the tank pressures employed and therefore no breakup (5A) was observed without gas injection. In the breakup illustrated (5B), approximately 2,400 uniform bubbles were injected per second, each having a volume of 6 X 10 cm at a pressure of 889 torr. As the entrained gas bubble leaves the orifice, it forms a bulge in the stream of oil directly beneath the opening. The gas in the bulge rapidly expands and breaks from the stream a discrete segment resembling a spherical cap, which subsequently expands into uneven sheets; this action taking place within several nozzle-opening diameters. Beneath this point, the sheet continues to expand and breaks into droplets along its outer edge and at thinner areas. For the example shown, at-about 25 cm below the nozzle, the sheet disintegrates 'into droplets with radii well below I/mm.

Studies were performed throughout the range of operating variables given below:

i a il law rats. L =10 0. 11m

Gas injection rate, G 0.2 3.0 l(STP)/min.

Pressure of injected gas, P, 200 1,900 torr Vacuum tank pressure, P, 0.5 15 torr Toluene content, X I to 10 vol. percent Toluene extracted, G 0.2 to 6.1 1(STP)/min. The results of these experiments indicated that the rate of toluene extraction (W) could be described by the formula 1 where K, is a constant for the system employed. In FIG. 6, the experimentally determined rate (W)is plotted as a function of (G G) X/P,, showing that for the range of variables investigated, the correlation is very good. It may, therefore, beseen that the rate of mass transfer is directly proportional to the rate of gas injection, increasing with increasing concentration of the volatile component and decreasing vacuum tank pressure (or increasing the ratio of vessel pressure to vacuum tank pressure). It has also been determined that for each liquid, there is an optimum range of bubble size. If ,the bubbles are too small, they will expend their energy in overcoming the surface tension forces of the liquid, rather than in the disintegration of the sheets of liquid. On the other hand, too large bubbles tend to burst prematurely, prior to their transferring their energy to the desired radial expansion of the liquid stream. However, it is essential that the bubble diameter be substantially smaller than the effective diameter of the orifice opening, to provide a liquid matrix with a discontinuous gas phase entrained therein.

Vacuum carbon degassing of steel With the exception of the addition of a gas injection pipe 5 as shown in FIG. 2, the apparatus employed was substantially that which is now employed in industry. In relation to steel, stream degassing processes are designed to remove dissolved hydrogen, nitrogen, and oxygen (as carbon monoxide) from steel by exposing the metal to a vacuum. As the steel enters the vacuum chamber, the stream is broken into a spray, and collected in a mold or ladle 4 at the bottom of the chamber. The violent disintegration of the steel stream into a spray, as it enters the vacuum chamber is considered to be the heart of the degassing process. It is generally believed that the stream is shattered by the rapid evolution and expansion of the gases dissolved within the steel (see, for example, N. Warner, J. Iron Steel Inst. 207, p. 44-50, 1969). In the instant process, gas is injected 5 in the form of bubbles, into the stream prior to its entrance in the vacuum chamber, this injection serving to 'both augment and control the degree of disintegration.

Argon was injected at metered rates of from 5 to l/min. STP through a heavy-walled fused silica tube. The drawn tip of the tube was one-eighth inch above the nozzle orifice and had an opening of approximately 0.060 inch. In several control runs, representative of prior art stream degassing methods, there was no gas injection.

The melts were prepared from lb. cylinders of AISI 1018 steel containing initially 0.15 0.20 percent C, 0.60 0.90 percent Mn and 0.10- 0.20 percent Si (all percentages by weight). Before the start of a pour, the melt was exposed to air for several minutes to insure that it was nearly saturated with oxygen. Iron oxide additions or longer exposures to air were used to produce lowered initial carbon contents of 0.098 0.16 percent. FIG. 7 shows-the percentage of oxygen removed as a function of gas injection rate. The significantly increasedv removal which may be achieved by employing bubble injection is clearly evident. In commercial practice even greater removal percentages would be achieved (both for no gas injection and with gas injection).- Thus, inthe experiments depicted, the distance between the nozzle and the level of metal in the mold 4, was approximately 2 ft., whereas in commercial size installations this distance is generally about 8 ft., thereby providing longer contact times with the vacuum.

For that portion of the results shown in FIG. 7, wherein gas injection rates of 15 1/min. were employed, the vacuum chamber pressure was varied from 6 to 180 torr in different runs. These runs are also plotted in FIG. 8, to show that at these high gas injection rates (ratio of gas volume to liquid metal volume of =4z1) the removal rate is substantially independent of chamber pressure. It may therefore be seen, that by the use of the gas injectionmethod as described herein, it is no longer necessary to attain a high vacuum to achieve substantial oxygen removal.

Since the disintegration of the liquid stream is not dejected gas.

pendent on saturating or dissolving a gas within the liquid, any non-deleterious gas may be employed for the purposes of spray formation. Thus, in addition to argon or other gases classified as inert, any gas may be employed which has negligible solubility in, and does not react detrimentally with, the liquid being treated. Thus, in many instances, inexpensive air injection may be employed.

We claim:

l. A method for controlling and augmenting the rate of extraction of volatile components of a liquid, which comprises:

teeming the liquid from the vessel in which it is contained, through a nozzle and into a zone maintained at a pressure substantially lower than the pressure of the liquid within said vessel, the crosssectional area of said nozzle being less than 0.5 the cross-sectional area of said containment vessel;

injecting a non-deleterious gas into said liquid prior to its passage through said nozzle, 'at a point wherein the liquid is at a pressure substantially higher than that of said lower pressure zone, said gas being injected at a rate by which spaced apart bubbles are formed, said bubbles being of a diameter substantially smaller than the effective diameter of the nozzle opening; and I urging said liquid through said nozzle with sufficient force so that the passage therethrough will be sufficiently rapid to prevent the bubbles from expanding to a substantial extent prior to the time the liquid has left the nozzle;.wher'eby the resultant expansion of the bubbles in said low pressure zone enhances the radial expansion and disintegration of the liquid into fine droplets, and collecting said droplets to form a degassed liquid pool.

2. The method of claim 1, wherein said point of injection is sufficiently proximate said nozzle, so as to cause a major portion of said bubbles to be entrained within the increment of liquid entering said nozzle.

3. The method of claim 2, wherein the rate of extraction is increased by increasing the flow rate of said in- 4. The method of claim 3, wherein said rate of extraction is increased by decreasing the radius of the injected bubbles.

5. The method of claim 2, wherein. said liquid is molten steel and said volatile components are gases dissolved therein.

6. The method of claim 5, wherein said nondeleterious gas is an inert gas. 

2. The method of claim 1, wherein said point of injection is sufficiently proximate said nozzle, so as to cause a major portion of said bubbles to be entrained within the increment of liquid entering said nozzle.
 3. The method of claim 2, wherein the rate of extraction is increased by increasing the flow rate of said injected gas.
 4. The method of claim 3, wherein said rate of extraction is increased by decreasing the radius of the injected bubbles.
 5. The method of claim 2, wherein said liquid is molten steel and said volatile components are gases dissolved therein.
 6. The method of claim 5, wherein said non-deleterious gas is an ''''inert'''' gas. 